[Stoves] Trials on TLUD Gas Burners - Burner Diameter

Wed Jul 23 18:07:08 CDT 2014

Dear Julien

This is a good and helpful report with a lot of practical experience built in.

On slide 7 it says “However, heating air decreases its density, so air velocity must increase to maintain the quantity of oxygen mixing with pyrogas.”

It is probably worth mentioning, because it is against common sense, that increasing the temperature reduces the density but increases the viscosity. Getting the right size hole is not quite as simple as a pro-rata change in area. That plus of course the shape of the entry hole changes the flow (a lot).  

@Paul S Anderson, what do you think of us putting together a chart with a diameter/flow plot for different levels of draft? 

Question: In slide 18, it gives the burn rate per unit time per unit volume. I can’t grasp that easily. Is the density somehow detectable in there? What is the mass loss rate in g/minute? While I realise that meaningful interpretation requires knowing the volume of fuel involved, I was trying to calculate the water mass out of the burn rate to see what the ‘real burn rate’ was. It is difficult to do that as the mass per unit time is not the metric.

Is there some other way you can report the burn rate for different fuel moisture levels?

It would be helpful, if you stick to the mg/minute/cm2 and first factor the mass loss raw data for the moisture content so it is mgDRY. In that way we can calculate the fuel burn rate and then, using the moisture level, calculate the true realisable energy (potential) value.

Performing this calculation sometimes gives surprises. From the look of the curves, the energy available drops as well as the burn rate. This is intuitive but not always the case. One thing to take a lesson from is the investigation which shows that 15% moisture fuel usually has lower PM emissions in most devices than 10% or 5%, but increasing it above 15% more sees the level rise.  For each stove type there is an optimum. 

For the TLUD’s with a different burner design, there is probably a different optimal moisture level.

Thanks for sharing the presentation.

Regards

Crispin

Hi Stoves;

In May, I reported to this listserv on some tests I had done with a menagerie of gas burners.

http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/2014-May/008665.html

My tinkering has continued, and some results are attached as a pdf that show some interesting features of ND-TLUDs.  For example, the best I could do for a turndown was about 2x, and turndown is very sensitive to primary air supply.

These tests follow from the earlier work were is was shown that placing a concentrator disk below the secondary air inlets did not result in a flame concentrated above the aperture in the disk.  Instead, pyrogas moved radially outward to the underside of the secondary air holes, and became entrained in the jets of secondary air. 

In addition, I was using gas burners that were 1.2 x larger than the diameter of the TLUD combustion chamber so as to get more space to manipulate secondary air, and more space for the gas flame to develop.  This work suggested that is was possible to increase the diameter of the gas burner even further.  The current trials put this to the test.  Here is an abstract of the study:

ABASTACT

Gas burners for a natural draft, top-lit updraft gasifier (ND-TLUD) were assessed for visible flame characteristics and their effect on gasification rate.  Two main design options were tested: diameter of the burner and preheating secondary air. Burners were tested on wood chip fuel that varied in moisture content, and primary air was adjusted to get very low to maximal rates of gasification.  Preheating secondary air had no visible effect on the gas flame nor did it alter gasification rate.  Increasing the size of the gas burner from 1x to 1.2x the diameter of the TLUD significantly improved gasification rate, reduced flame height, and reduced visible smoke production.  Further increasing burner diameter to 1.5x TLUD diameter didn’t cause a further increase gasification rate, and made the reaction unstable at low rates of primary air and high fuel moisture contents.  Increasing the diameter of burners to at least 1.2x looked very promising for improving TLUD operation.  Burners of 1.5x may be suitable for TLUDs that only run at high gasification rates.  Larger diameter burners will likely cause less air pollution, and should be tested for CO and particulate emissions.

Increasing the size of gas burners could be tested with some of Jock Gill's ideas for increasing flame temperature in the burner.

http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/2014-May/008843.html 

All the best,

Julien.

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